Magnetic sensors, such as Hall Effect sensors and magnetoresistive sensors, are well known for use in measuring the position of an element. Generally, a magnet is used to create a magnetic field which is measured by an IC (integrated circuit) containing a magnetically sensitive feature. The magnet is connected to the element to be measured and moves relative to the IC. The changing magnetic field at the IC is converted into an output signal proportional to the movement.
Magnetic based sensors have three major limitations. First, the magnet can lose strength over time and temperature, which can lead to error in the indicated position. These losses can be caused by exposure to temperature which allows some meta-stable domains to rotate or by corrosion which changes the metallurgy, or by bad processing. Secondly, the IC can drift over time and temperature or the IC can fail outright. Thirdly, existing structures for magnetic sensors are very sensitive to small changes in magnetic field that can occur with small mechanical misalignment.
An example of an effective position sensor comprises a radially magnetized permanent ring magnetic longitudinally split into opposed first and second portions with the North pole of each portion aligned in reverse orientation relative to each other and being mounted in a yoke of magnetic material for rotation with the yoke. A generally coaxial cylindrical stator, longitudinally split into first and second portions and spaced from one another by a selected secondary air gap, is disposed within and spaced from the ring magnet forming a primary air gap. A Hall sensor is disposed within the secondary air gap between the first and second stator portions. This arrangement provides a nearly linear electrical output signal proportional to the angular position of the yoke mounting the ring magnet and is not sensitive to misalignment between the rotating and stationary members. For further details, reference can be made to U.S. Pat. No. 5,789,917, the subject matter of which is incorporated herein by this reference.
Although sensors made according to the teachings of this patent are very effective, there is a limitation in their use. That is, over time the strength of the magnet decays and the Hall sensor reflects this as an angular rotation. Position sensors of this type typically use a samarium cobalt magnet. After 3,000 hours at 150 degrees C., such magnets typically experience a decrease of 2–4% in remanence. This decaying field causes a decrease of the output and thus an error in the angular position read out. Although errors of this magnitude may be acceptable in certain applications, there are many other applications in which such errors cannot be tolerated.
Attempts have been made to address this problem by running a temperature stabilization cycle on the magnets. While this has some beneficial effect in reducing aging, it does not eliminate it. Further, in order to obtain the 1–2 ppm defect level required for highest quality, stable magnets, one must have nearly perfect process controls.